1. Field of the Invention
The present invention relates to an image heating apparatus for heating a toner image formed on a recording material, and more particularly to an image heating apparatus adapted for use as a fixing apparatus to be mounted on an image forming apparatus such as a copying apparatus or a printer.
2. Related Background Art
As a fixing apparatus equipped in an image forming apparatus of electrophotographic process or electrostatic recording process, there has been widely employed a heat fixing device of so-called heat roller type, in which a recording material bearing an unfixed toner image is passed through a nip portion formed by a fixing roller and a pressure roller which are rotated in a mutually pressed state, thereby fixing the unfixed toner image onto the recording material as a permanent image.
On the other hand, a reduction in the electric power consumption is strongly desired as a recent environmental issue while a high image quality and a high speed in the image output are requested from market demands. Thus, various improvements are being tried in the heat fixing apparatus of the aforementioned heat roller type, in order to meet such requirements for the decreased electric power consumption and for the high image quality and the high speed.
The present applicant has proposed, in Japanese Patent Application Laid-Open No. 2003-182367, an image heating apparatus capable, as a fixing apparatus, of achieving an image output with a high image quality at a high speed, while maintaining a reduced electric power consumption and a shortened heating time. This fixing apparatus, as shown in FIG. 11, is provided with a fixing roller 20 having an elastic layer 22, a heating member 24 in contact with an external surface of the fixing roller 20 thereby forming a heating nip N, and a pressure member 30 maintained in a pressurized contact with the fixing roller thereby forming a fixing nip portion (conveying rip portion) M, in which a recording material P bearing an unfixed toner image t is pinched and conveyed to achieve heat fixation (apparatus of this type being hereinafter called “external heating type”).
Also a heater 26 equipped on the heating member 24 is of a plate shape of a low heat capacity, of such a type generating heat in sliding contact with the heating nip N. Such configuration allows to achieve a higher energy density at the heating nip N in comparison with a structure of heating the surface of the heating roller by a heat roller in contact with the surface of the fixing roller, thereby enabling to promptly heating the surface of the fixing roller.
Also the contact of the elastic member 22 of the fixing roller with the recording material P or the toner t is equivalent to that in a heat roller type having an elastic layer, as employed in a prior high-speed apparatus, so that a high image quality can be maintained even for a higher speed in the image forming apparatus. Thus, such system is capable of simultaneously satisfying all the requirements, such as a reduction in the start-up time, a reduction in the electric power consumption, and a high-quality image output in a high-speed operation.
However, in the fixing apparatus shown in FIG. 11, in the heat fixation of a recording material, the toner on the recording material may be offset to the fixing roller, and such offset toner is deposited onto the surface of the heating member by the frictional contact between the heating member and the fixing roller. As the heat fixing operation is repeated, such offset toner is accumulated on the surface of the heating member, and, upon exceeding a certain amount, is peeled from the surface of the heating member and transferred onto the recording material thereby forming an image defect.